Phyllosilicate composites containing mica

ABSTRACT

Disclosed is a mica paper composite and a process for making the mica paper composite. Articles comprising the mica paper composite are also disclosed.

FIELD OF THE INVENTION

Mica paper composites and processes for making them are provided.Articles comprising the mica paper composites, such as photovoltaiccells, are also provided.

BACKGROUND

Photovoltaic cells, which receive light and convert the light intoelectric energy, are made by depositing various layers of materials on asubstrate. The most common substrate material used in the manufacture ofthin film Cu(In,Ga)Se (CIGS) photovoltaic cells is glass because glassprovides a good balance of properties at moderate cost. In particular,glass provides good mechanical support; is thermally and chemicallystable to the processes used to deposit various layers of the thin filmphotovoltaic cell onto the substrate; is electrically insulating; andprovides excellent barrier properties to protect the water andoxygen-sensitive layers of the photovoltaic cell. In addition, glass hasa smooth surface, which enables the surface of a film that is placed onthe glass to be made relatively smooth in turn.

Glass substrates also have some disadvantages. They are heavy, prone tobreakage, and generally too rigid to be used in potentially moreeconomical roll-to-roll processes. These disadvantages have motivatedthe search for alternative substrates. Metal foils can be used assubstrates, but have the distinct disadvantage that they areelectrically conductive and are also heavy. Organic polymers, such aspolyimides, are amenable to use in roll-to-roll processes and can beweight-saving substrates in many applications, but they do not havesufficient thermal and dimensional stability at the high temperatures,for example above 500° C., which are needed to realize higherphotovoltaic efficiencies for CIGS based devices. The use of mica as asubstrate has also been reported.

U.S. Pat. No. 7,663,056 discloses a chalcopyrite type solar cell havinga mica aggregate substrate formed by binding mica particulates with aresin.

US Published Patent Application No. 2009/0133749 discloses achalcopyrite solar cell having a mica substrate or a laminated micasubstrate.

US Published Patent Application No. 2009/0202806 discloses an inorganiclayered compound film comprising an oriented denatured clay andexhibiting high water resistance, excellent pliability, excellent gasbarrier properties and high heat resistance. The clay used in thedenatured clay may be mica. The denatured clay may be reacted with asilylating agent.

US Published Patent Application No. 2009/0205715 discloses a solar cellwhich includes a substrate of mica or material containing mica; anintermediate layer for smoothing or planarizing a surface of thesubstrate, which is formed on the substrate; a binder layer formed onthe intermediate layer; a metallic lower part electrode layer formed onthe binder layer; a p-type light absorbing layer formed on the metalliclower part electrode layer, and made of chalcopyrite based material; ann-type buffer layer formed on the light absorbing layer; and an n-typetransparent electrode layer formed on the buffer layer.

Mica substrates that are flexible, tolerant of the high temperaturesused to create the photoactive layers, inexpensive, and suitable for usein roll-to-roll processes continue to be sought. Mica substrates whichfurthermore have improved or suitable tear initiation resistance,improved water resistance, and a thermal expansion coefficient similarto that of the metal used in a back electrode of a photovoltaic cell areespecially sought.

SUMMARY

In one aspect, the present invention is a mica paper compositecomprising a phyllosilicate component having a general Formula (V):

[KMg₃(OH)₂[Si₃AlO₁₀]]_(j)[M^(m)O_((n-g))(OH)_(g)]_((1-j))  (V)

wherein:1) M is selected from Si, Al, Ti, Zr, and mixtures thereof, with theproviso that:

-   -   i) when M is Si, Ti, or Zr, the value of m is 4; and    -   ii) when M is Al, the value of m is 3;        2) n is a value represented by m/2;        3) g and (n-g) represent molar proportions, where g is a value        in the range of 0 to n;        4) j represents the weight fraction of mica in the mica paper        composite; and,        5) the value of j is in the range of 0.995 to 0.8.

DETAILED DESCRIPTION OF THE INVENTION

Applicants specifically incorporate the entire contents of all citedreferences in this disclosure. As used herein, when an amount,concentration, or other value or parameter is given as either a range,preferred range, or a list of upper preferable values and lowerpreferable values, this is to be understood as specifically disclosingall ranges formed from any pair of any upper range limit or preferredvalue and any lower range limit or preferred value, regardless ofwhether ranges are separately disclosed. Where a range of numericalvalues is recited herein, unless otherwise stated, the range is intendedto include the endpoints thereof, and all integers and fractions withinthe range. It is not intended that the scope of the invention be limitedto the specific values recited when defining a range.

DEFINITIONS

The following definitions are used in this disclosure:

“Mica paper” refers to a thin sheet of mineral derived fromphyllosilicates of the mica group.

“Impregnating” refers to a process in which a substrate, for examplemica paper, contacts a liquid composition.

“Dip coating” refers to a process in which a substrate is immersed in aliquid composition and then removed from the liquid composition.

“Gravure coating” refers to a process in which a substrate such as micapaper is contacted with a liquid composition in a roll to roll process.The roll surface is engraved with a pattern of cells which provide aspecific coating volume. The roll is mounted in bearings and rotatespartially submerged in a coating pan which contains the liquidcomposition to be coated onto the substrate. Rotation of the roll allowsthe substrate to pick up the coating, which is doctored (pre-metered) bya flexible steel blade as the roll rotates toward the contact point withthe substrate. Standard gravure methods (reverse or direct) use abacking roll, usually rubber covered, having about the same diameter asthe engraved roll. The substrate is trapped (nipped) between theengraved roll and the backing roll.

In “direct gravure coating”, the rotation of the engraved cylinder is inthe same direction as the travel direction of the substrate.

In “reverse gravure coating”, the rotation of the engraved cylinder isopposite to the travel direction of the substrate and the liquid coatingapplied to the substrate experiences shear.

Micro Gravure™ is a reverse, kiss gravure coating method in which abacking roll which traps the substrate against the engraved roll isabsent.

“Slot die coating” refers to a method of applying a liquid compositionto a web, for example a web consisting of mica paper. The liquid isforced out from a reservoir through a slot by pressure and transferredto a moving web. In practice, the slot is generally much smaller insection than the reservoir, and is oriented perpendicular to thedirection of web movement.

“Slot die” coating refers to a method of coating with a die “against” aweb of mica paper, in which the mica paper substrate is actuallyseparated from the web by a cushion of the liquid composition beingcoated.

“Bar coating” refers to a coating method in which a bar with a clearanceor gap equivalent to the desired wet coating thickness is contacted withthe mica paper substrate. In this process, the bar or the web (micapaper) can be conveyed in a constant direction. A bead of a liquidcomposition is applied to the bar prior to the conveyance of the bar orthe mica paper.

“Rod coating” refers to a process which is similar to bar coating,except that a Mayer rod is used in place of the bar. The “Mayer” rod isa stainless steel rod that is wound tightly with stainless steel wire ofvarying diameter. The rod is used to doctor off the excess coatingsolution and to control the wet coating thickness. The wet thicknessafter doctoring is controlled by the diameter of the wire used to windthe rod.

“Spray coating” refers to a process whereby a liquid composition isatomized and applied to the surface of a substrate as small droplets ofliquid.

“Spin coating” refers to a process in which coatings are applied to flatsubstrate surfaces by placing a fluid coating solution on the substrateand then rotating the substrate at high speed to spread the coatingsolution by centrifugal force.

“Planarizing” refers to a process for reducing the surface roughness ofa substrate such as a mica paper composite. As used herein, planarizingincludes the process of applying a coating to a substrate as well as theprocess of reducing the surface roughness of the substrate through theuse of pressure and/or heat.

In the processes described herein, a mica paper is contacted with acomposition comprising at least one alkoxide, wherein the alkoxide isselected from the group consisting of an aluminum alkoxide, a siliconalkoxide, a titanium alkoxide, and a zirconium alkoxide, to form analkoxide-treated mica paper, which is then dried and calcined to obtaina mica paper composite. The mica paper composite may be used tofabricate various articles, including a photovoltaic cell.

Natural and synthetic micas are well known minerals, see, for examplethe entry in the Kirk-Othmer Concise Encyclopedia of Chemical Technology(John Wiley & Sons, 1985, p. 759-760). Mica paper useful in the presentinvention comprises at least one phyllosilicate of the mica group andcontains no polymeric binders. The phyllosilicate may comprisemuscovite, phlogopyte, illite, zinnwaldite, lepidolite, paragonite,biotite, fluorophlogopite, or combinations thereof. In one embodiment,the mica paper comprises muscovite, illite, or a combination thereof.Mica paper sheets having typical thickness between 0.25 mil and 4 milscan be formed from mica-containing pulp by conventional paper processes.In one embodiment of the present invention, a mica paper sheet thicknessin the range of about 0.5 mil to about 3.0 mils is preferred. Mica papersuitable for use herein may be obtained from commercial sources.

In one embodiment of the process to obtain a mica paper composite, thecomposition comprises decomposition products derived from at least onealkoxide of Formula (I), Formula (II), or mixtures thereof,

Q_(x)Al(OR)_(y)  (I)

Q_(p)M″(OR)_(q)  (II)

wherein:

1) Q is hydrogen or an organic group having from 1 to 20 carbon atoms;and

-   -   i) x is 0, 1, or 2; and    -   ii) y is 1, 2, or 3, with the proviso that x+y=3;

2) M″ is Si, Ti, or Zr;

-   -   i) p is 0, 1, 2, or 3; and    -   ii) q is 1, 2, 3, or 4, with the proviso that p+q=4; and

each OR moiety independently has a structure OR′, OR″, OR′″, or OR″″wherein the radicals R′, R″, R′″, and R″″ are independently anunsubstituted or substituted alkyl group having from 1 to 20 carbonatoms, an unsubstituted or substituted aromatic group having from 6 to18 carbon atoms, or an unsubstituted or substituted cycloaliphatic grouphaving from 6 to 18 carbon atoms.

In one embodiment, Q represents an organic group having from 1 to 10carbon atoms. Suitable organic groups include methyl, ethyl, butyl, andphenyl groups. In one embodiment, the radicals R′, R″, R′″, and R″″ areindependently an unsubstituted or substituted alkyl group having from 1to 10 carbon atoms, an unsubstituted or substituted aromatic grouphaving from 6 to 10 carbon atoms, or an unsubstituted or substitutedcycloaliphatic group having from 6 to 10 carbon atoms.

Suitable alkyl groups include substituted and unsubstituted, saturatedand unsaturated, alkyl groups such as methyl, ethyl, n-propyl,iso-propyl, n-butyl, iso-butyl, sec-butyl, tert-butyl, amyl, pentyl,hexyl, heptyl, octyl, nonyl, vinyl, propenyl, acyloyloxy,methacryloyloxy, and acrylic groups. Examples of suitable aromaticgroups include phenyl and napthyl groups. Suitable cycloaliphatic groupsinclude substituted and unsubstituted, saturated and unsaturated,cycloaliphatic groups such as cyclohexyl, methylcyclohexyl,dimethylcyclohexyl, ethylcyclohexyl, propylcyclohexyl, andbutylcyclohexyl groups.

Examples of alkoxides suitable for use in the present invention includetetraethylorthosilicate, titanium isopropoxide, titanium 2-ethylhexanolate, titanium n-butoxide, titanium ethoxide, zirconiumn-propoxide, zirconium n-butanolate (zirconium n-butoxide),vinyltrimethoxysilane, vinyltriethxoysilane, and3-methacryloxypropyltrimethoxysilane. Titanium isopropoxide andtetraethylorthosilicate are preferred. The alkoxides may be obtainedfrom commercial sources or, alternatively may be prepared by knownmethods.

Optionally, the composition comprising at least one alkoxide maycomprise an organic solvent in which the at least one alkoxide ofFormula (I) and/or Formula (II) is soluble. The solvent may be analcohol, acetate, amide, aromatic hydrocarbon, aliphatic hydrocarbon,chlorinated aliphatic hydrocarbon, or a mixture thereof. Suitablesolvents include alcohols such as methanol, ethanol, propanols, andbutanols; acetates such as ethyl acetate; amides such asdimethylacetamide and dimethylformamide; aromatic hydrocarbons such asbenzene, toluene, and xylenes; aliphatic hydrocarbons such as hexanesand heptanes, and chlorinated aliphatic hydrocarbons such asdichloromethane. The choice of solvent should be made judiciously toavoid unwanted byproducts and/or decrease the amount of availablealkoxide. For example, the organic solvent should be substantiallyanhydrous or water free in order to avoid hydrolysis of the alkoxide.

The contacting of the mica paper with a composition comprising at leastone alkoxide may be performed by any suitable means whereby the micapaper imbibes sufficient liquid so that at least some of the compositioncomprising at least one alkoxide is absorbed into its pores and onto itssurface. For example, contact can be by impregnating, dip coating,direct gravure coating, reverse gravure coating, direct microgravurecoating, reverse microgravure coating, slot die coating, bar coating,rod coating, spray coating, spin coating, or a combination thereof. Thecontacting of the mica paper with a composition comprising at least onealkoxide is carried out for a time sufficient to introduce the desiredamount of alkoxide to the mica paper.

The duration of the contact typically may be from about 0.01 seconds toabout 5 hours. The contact period can be chosen using various factors toinform the choice. For example, the efficiency of the imbibing of theliquid or economic considerations may influence the duration of contact.A shorter period of time may be preferable for practical and/or economicreasons. Typically a period of contact may be from about 0.1 seconds toabout 60 minutes. The contacting of the mica paper with a compositioncomprising at least one alkoxide may be performed at a relatively hightemperature for a relatively short period of time, or at a lowertemperature for a longer period of time.

For the contacting of the mica paper with a composition comprising atleast one alkoxide, the temperature, contacting time, type of micapaper, thickness of the mica paper, specific alkoxide used, alkoxideloading, the total pore volume, and the surface area of the paper arerelated; thus, these variables may be adjusted as necessary to obtain anoptimal product.

For the contacting of the mica paper with a composition comprising atleast one alkoxide, the temperature, contacting time, type of micapaper, thickness of the mica paper, specific alkoxide used, alkoxideloading, the total pore volume, and the surface area of the paper arerelated; thus, these variables can be adjusted with consideration givento the effect that such adjustment will have on the product obtained.

After contacting the alkoxide composition, the alkoxide-treated micapaper is dried, that is, the solvent is evaporated from the treatedpaper. For example the treated paper can be dried in air at roomtemperature, or by heating to temperatures up to about 200° C. The driedalkoxide-treated mica paper is then calcined to obtain a mica papercomposite. The calcining step is performed by heating the driedalkoxide-treated mica paper for a time and at a temperature sufficientto decompose the at least one alkoxide to form decomposition productscomprising oxides, oxyhydroxides, or a combination thereof. In oneembodiment, after calcining the mica paper composite contains from about0.5 weight percent to about 30 weight percent alkoxide decompositionproducts comprising oxides, oxyhydroxides, or a combination thereof. Inother embodiments the calcined mica paper composite may contain, forexample, from about 0.5 weight percent to about 25 weight percent, orfrom about 1 weight percent to about 20 weight percent, or from about0.5 weight percent to about 15 weight percent, or from about 0.5 weightpercent to about 10 weight percent, or from about 0.5 weight percent toabout 5 weight percent of decomposition products. For the calciningstep, temperatures greater than approximately 300° C. are required for atime period ranging between 1 minute and 12 hours. A preferredtemperature range is about 500° C. to about 700° C. for a time period ofabout one minute to about one hour in air. An example of a suitablecalcining method is presented in the Examples.

The present process may optionally further comprise a planarization stepto improve the surface smoothness (that is, reduce the surfaceroughness) of the mica paper composite. Planarization of the micasurface can be necessary because irregularities in the surface of themica paper composite can be reproduced in layers deposited on top of themica paper composite, for example in an electrode layer and in a lightabsorption layer, and can negatively affect a CIGS solar cell, forexample by decreasing the open circuit voltage. If carried out, theplanarization step may be performed after drying (before calcining) orafter calcining.

Planarization processes are known, particularly for hard, non-poroussurfaces. Typically, a planarizing coating can be applied by a number ofmethods, including coating liquid precursors by dip coating, spraycoating, gravure, reverse gravure, microgravure and spin coatingprocesses. The planarizing layer can also be applied by otherconventional methods including laminating, physical vapor deposition(evaporative deposition, sputter deposition, pulsed laser deposition orlaser ablation), or chemical vapor deposition. Planarization can also beperformed by any method which applies pressure and optionally heat tothe surface of the mica paper, the mica paper composite, or anuncalcined precursor. Planarization methods used in paper processingtechnology (calendering or thermal calendering) can be used. Byadjusting the number of rolls, the number of nips, nip pressure and thelike, it is possible to planarize both sides of the mica containingpaper by passing the paper through nip rollers in a continuous, roll toroll process.

The planarization can be performed by, for example, a method to heat thematerial after coating with the planarizing solution and then to applypressure on the coated surface, a method to carry out a smoothing rolltreatment after the coating step, a method to apply a mirror finish inthe drying process, a method to treat the coated material by passing itthrough a multicylinder dryer roll or a Yankee dryer roll that isplanarized, and other methods.

For example, for the treatment carried out by applying pressure and heaton the surface of the mica paper composite (or of an uncalcinedprecursor), planarization methods used in the paper processingtechnology, such as thermal calender and super calender treatments, canbe used. With the thermal calender treatment, it is possible to achieveeasy planarization due to the removal of water contained in the micapaper composite, since heat can be applied in addition to theplanarization by the roll pressure. With the super calender treatment,by adjusting the number of rolls, the number of nips and/or nippressure, it is possible to planarize both sides of the mica at the sametime to achieve two surfaces with equivalent surface flatness. In eithercase, it is required that the surface of the calender roll used has acertain degree of flatness and is preferably 100 nm or less in terms ofsurface roughness Ra. The roll surface with such surface flatness can beplanarized by mirror finish, polishing, or other methods.

In addition, it is also possible to use a general heat press machine asa means of applying heat and pressure. As the heat press machine, anapparatus which is capable of applying heat and pressure by the use of aroll, a batch heat press machine, or the like can be used. In order toachieve a continuous mica paper composite, it is preferable to use anapparatus which applies heat and pressure by the use of a roll. It isalso possible to cut out a piece of mica paper composite of a certainsize from the continuous mica paper composite and subject the piece to afurther batch-wise pressing treatment using a heat press machine inorder to improve the surface planarization. Part of the heat pressmachine which contacts the surface of the mica paper composite, that is,the press plate, preferably has a surface roughness Ra of 100 nm orless. The mica paper composite will have a uniform film thickness and asmall surface roughness due to the application of pressure by a heatpress machine.

The above treatment by a heat press machine is preferably carried out ina vacuum. By performing the treatment in a vacuum, it is possible toreduce the extent of voids inside a mica paper composite and achieve adenser mica paper composite.

As another method to planarize the surface, a method employing asmoothing roll treatment and other methods can be adopted. For example,immediately after coating the substrate surface by an appropriatecoating method, it is possible to improve surface flatness by smoothingthe surface using a smoothing roll. A heated smoothing roll can also beused.

In the present invention, it is also possible to combine theabove-mentioned planarization methods or other planarization methods foruse. Moreover, by appropriately adjusting the roll materials used, theapplied pressure, the heating temperature, the feeding speed and thefrequency, and the planarizing solution, the desired degree of flatnesscan be achieved.

Alternatively, the surface smoothness of the mica paper composite may beimproved by application of heat and/or pressure as described above butwithout first coating the mica paper composite with a planarizingsolution. Calendering is an example of such a method of improving thesurface smoothness. The present process may optionally further comprisea step of calendering the mica paper before contacting it with acomposition comprising at least one alkoxide, calendering the driedalkoxide-treated mica paper, or calendering the mica paper compositeafter calcining.

One embodiment of the present invention is a mica paper compositecomprising oxides, oxyhydroxides, or a combination thereof which arederived from decomposition of an aluminum alkoxide.

One embodiment of the present invention is a mica paper compositecomprising oxides, oxyhydroxides, or a combination thereof which arederived from decomposition of a silicon alkoxide, a titanium alkoxide, azirconium alkoxide, or a combination thereof.

In another embodiment, the present invention is a mica paper compositecomprising the decomposition product of an alkoxide of Formula (I):

Q_(x)Al(OR)_(y)  (I)

In another embodiment, the present invention is a mica paper compositecomprising the decomposition product of an alkoxide of Formula (II):

Q_(p)M″(OR)_(q)  (II)

One embodiment of the present invention is a mica paper compositecomprising a composition represented by Formula (III):

[[A_(a) ^(a′)B_(b) ^(b′)C_(c) ^(c′)D_(d)^(d′)]^(z)(Si_(4-x)Al_(x)O₁₀)^(y)[OH_(1-v)F_(v)]₂]_(j)[M^(m)O_((n-g))(OH)_(g)]_((1-j))  Formula(III)

wherein:

1) A, B, C, and D are metals independently selected from Li, Na, K, Rb,Cs, Mg, Ca, Sr, Ba, Fe, Ti, Cr, Al, and Mn;

2) a, b, c, d, x, and v represent molar proportions wherein

-   -   i) x is a value in the range of 0.13 to 2;    -   ii) v is a value in the range of 0 to 1; and    -   iii) a, b c, and d are values within the range of zero (0) and        the quantity defined by (x+6)/Ox, wherein Ox is the        corresponding oxidation state a′, b′, c′ or d′;

3) a′, b′, c′, d′ and m represent oxidation states having integer valueseach independently in the range of from +1 to +6;

with the proviso that:

-   -   (i) when the molar proportion (a, b, c, or d) is multiplied by        the oxidation state (a′, b′, c′, or d′) of the corresponding        metal (A, B, C, or D) and the products of the mathematical        operations are all added, the result equals the value of z, that        is,

a·a′+b·b′+c·c′+d·d′=z

-   -   ii) y=−(x+4)    -   iii) z=2−y;

4) M is selected from Si, Al, Ti, Zr, and mixtures thereof;

with the proviso that

-   -   i) when M is Si, Ti, or Zr, the value of m is 4; and    -   ii) when M is Al, the value of m is 3;

5) n is a value represented by m/2;

6) g and (n-g) represent molar proportions, where g is a value in therange of 0 to n;

7) j represents the weight fraction of mica in the mica paper composite;and

8) the value of j is in the range of 0.995 to 0.7.

In one embodiment, a mica paper composite comprises a compositionrepresented by Formula (IV). Formula (IV) is a version of Formula (III),wherein v is 0, n is 2, given by the general formula:

[[A_(a) ^(a′)B_(b) ^(b′)C_(c) ^(c′)D_(d)^(d′)]^(z)(Si_(4-x)Al_(x)O₁₀)^(y)[OH]₂]_(j)[MO_((2-g))(OH)_(g)]_((1-j))  Formula(IV)

wherein:

1) A, B, C, and D are metals independently selected from Li, Na, K, Rb,Cs, Mg, Ca, Al, and Sr;

2) a, b, c, d, and x represent molar proportions wherein

-   -   i) x is a value selected from the range of 0.5 to 1; and    -   ii) a, b c, and d are values within the range of zero (0) and        the quantity defined by (x+6)/Ox, wherein Ox is the        corresponding oxidation state a′, b′, c′ or d′.

3) a′, b′, c′, d′, represent oxidation states are integer values eachindependently in the range of +1 to +6;

with the proviso that:

-   -   (i) when the molar proportion (a, b, c, or d) is multiplied by        the oxidation state (a′, b′, c′, or d′) of the corresponding        metal (A, B, C, or D) and the products of the mathematical        operations are all added, the result equals the value of z, that        is,

a·a′+b·b′+c·c′+d·d′=z

-   -   ii) y=−(x+4)    -   iii) z=2−y;

4) M is selected from Si, Ti, Zr, and mixtures thereof;

5) g and (2-g) represent molar proportions, where g is a value in therange of 0 to 2;

6) j represents the weight fraction of mica in the mica paper composite;and

7) the value of j is in the range of 0.995 to 0.7.

In one embodiment, the mica paper used to produce the mica papercomposite comprises muscovite, and the mica paper composite comprises acomposition represented by Formula (V)

[KMg₃(OH)₂[Si₃Al O₁₀]]_(j)[M^(m)O_((n-g))(OH)_(g)]_((1-j))  Formula (V)

Formula (V) is a version of Formula (III) in which:1) A is K; a is 1; a′ is 1; B is Mg; b is 3; b′ is 2; c is 0; d is 0; xis 1; and v is 0;2) M is selected from Si, Al, Ti, Zr, and mixtures thereof, with theproviso that:

-   -   i) when M is Si, Ti, or Zr, the value of m is 4; and    -   ii) when M is Al, the value of m is 3;        3) n is a value represented by m/2;        4) g and (n-g) represent molar proportions, where g is a value        in the range of 0 to n;        5) j represents the weight fraction of mica in the mica paper        composite; and        6) the value of j is in the range of 0.995 to 0.8.

In one embodiment, the mica paper used to produce the mica papercomposite comprises illite, and the mica paper composite comprises acomposition represented by Formula (VI)

[K_(0.65)Al₂[Al_(0.65)Si_(3.35)O₁₀](OH)₂]_(j)[M^(m)O_((n-g))(OH)_(g)]_((1-g))  Formula(VI)

Formula (VI) is a version of Formula (III) in which:1) A is K; a is 0.65; a′ is 1; B is Al; b is 2; b′ is 3; c is 0; d is 0;x is 0.65, and v is 0;2) M is selected from Si, Al, Ti, Zr, and mixtures thereof;

with the proviso that:

-   -   i) when M is Si, Ti, or Zr, the value of m is 4; and    -   ii) when M is Al, the value of m is 3;        3) n is a value represented by m/2;        4) g and (n-g) represent molar proportions, where g is a value        selected from the range of 0 to n;        5) j represents the weight fraction of mica in the mica paper        composite, and the value of j is selected from the range of        0.995 to 0.8.

In one embodiment, the mica paper composite is made by the processdescribed herein. In one embodiment of the invention, a photovoltaiccell comprises the mica paper composite.

The mica paper composite disclosed herein is an insulator having anextremely high resistance, for example ranging from 10¹² to 10¹⁶ ohms.The mica paper composite is also tolerant of high temperature, forexample temperatures in the range of about 600° C. to about 800° C., andis highly resistant to water, acids, alkalis, and H₂Se gas. In addition,the mica paper composite is light in weight, tear resistant, and highlyflexible, which enables it to be wound in a rolled form, for example ina roll-to-roll process for manufacturing photovoltaic cells. Havingthese characteristics, the mica paper composites described herein areuseful as substrates for layered assemblies or other devices. Forexample, the mica paper composites can be used in an assembly comprisinga mica paper composite and a conductive electrode layer, wherein theelectrode layer comprises Mo, W, Cr, or mixtures thereof. Such anassembly may further comprise a light absorber layer, which mayoptionally comprise Cu, In, Ga, and Se. Alternatively, the lightabsorber layer may optionally comprise Cu, Zn, Sn, and S. The assemblycomprising a light absorber layer may further comprise a buffer layer,such as CdS. An example of a device comprising such an assembly is aphotovoltaic cell.

Thin-film photovoltaic (PV) cells typically comprise a substrate, aconductive (electrode) layer, a light absorber layer of photovoltaicmaterial, an n-type buffer layer thin film formed by InS, ZnS, CdS orthe like, a transparent conducting oxide (TCO) layer formed fromaluminum doped ZnO or the like, and a metal grid top contact layer. Someembodiments may optionally contain one or more layers selected frombuffer layers and interconnect layers.

In the photovoltaic cell of this invention, the substrate is a micapaper composite prepared as described above. The conductive layer is ametal layer that has been deposited on the mica paper composite layer.This provides a flexible inorganic substrate for photovoltaic cells thatis light-weight and thermally stable at 450-700° C.

The conductive layer comprises Mo, W, Cr, or mixtures thereof. Theconductive metal is typically deposited to a thickness of 200-1000 nm bysputtering onto the mica paper composite layer. Preferably, theconductive layer is uniform in thickness and pin-hole-free.

The photovoltaic material for the light absorber layer is selected fromthe group consisting of amorphous silicon (a-Si), cadmium telluride(CdTe), copper indium (gallium) di-selenide/sulfide (CIS/CIGS), CuInSe₂,CuInS₂, CuGaSe₂, CuInS₂, CuGaS₂, CuAlSe₂, CuAlS₂, CuAlTe₂, CuGaTe₂,Cu₂ZnSnS₄, Cu₂ZnSnSe₄, and combinations thereof. The layer ofphotovoltaic material is deposited on the conductive layer. In oneembodiment, the light absorber layer comprises Cu, In, Ga, and Se. Inone embodiment, CIGS is applied by co-evaporation of Cu, In and Ga inthe presence of Se vapor 600° C., followed by chemical bath depositionof CdS. In another embodiment, CZTS (copper zinc tin sulfide) is appliedby printing an ink of precursor particles on the conductive layer,followed by annealing at 600° C. The annealing step is followed bychemical bath deposition of CdS.

The TCO layer typically includes mixtures or doped oxides of In₂O₃,SnO₂, ZnO, CdO, and Ga₂O₃. Common examples in PV cells include ITO(In₂O₃ doped with about 9 atomic % Sn) and AZO (ZnO doped with 3-5atomic % Al). In one embodiment, ZnO is sputter deposited onto the layerof photovoltaic material.

The metal grid top contact layer typically comprises a patterned metallayer, where the metal is selected from the group consisting of copper,silver, gold, nickel, chromium, aluminum and mixtures thereof. In oneembodiment, e-beam evaporation is used to deposit Ni/Al grids.

In some embodiments, an anti-reflective coating is deposited on themetal grid top contact layer. Suitable anti-reflective coatings includeMgF₂.

The structure of a-Si and nc-Si solar cells is commonly p-i-n for asingle cell, wherein “n” refers to n-type Si, “i” refers to insulatingSi, and “p” refers to p-type Si. Tandem cells with higher efficiency areproduced by stacking this basic cell and optimizing the absorption ofthe stack.

Thin-film silicon solar cells typically comprise a TCO layer, a p-typeSi alloy layer, an i-Si alloy layer, an n-type Si alloy layer, a bufferlayer, a metal layer and a substrate. Thin-film silicon solar cells andamorphous silicon-based solar cells—including additional configurationssuch as, for example, multiple junction solar cells—are described in,for example, Handbook of Photovoltaic Science and Engineering, AntonioLuque and Steven Hegedus (2003), chapter 12. In the thin-film solarcells of this invention, the metal layer comprises Mo, W, Cr, ormixtures thereof and the substrate is a mica paper composite.

Amorphous or nanocrystalline Si is usually an alloy with hydrogen, i.e.,a-Si:H or nc-Si:H. Doping n-type or p-type can be accomplished usingcommon dopants used for crystalline Si. Suitable p-type dopants includeGroup III elements (e.g., B). Suitable n-type dopants include Group Velements (e.g., P). Alloying with Ge or C can also be used to change theoptical absorption characteristics and other electrical parameters.

The buffer layer is typically a transparent, electrically insulatingdielectric. Suitable materials include CdS, ZnSe, (Zn,Mg)O, In(OH)₃,In₂S₃, In₂Se₃, InZnSe_(x), SnS₂, ZnO, Ga₂O₃, SnO₂, and Zn₂SnO₄.

In one embodiment, the photovoltaic cell may be laminated to top andbottom sheets using an encapsulant layer. The top and bottom sheets canbe glass or polymer films that protect the photovoltaic material fromoxygen and water. Ethylene copolymers such as EVA (ethylene vinylacetate) are suitable encapsulants.

Suitable glass top sheets have high transmission (>80%) throughout thesolar spectrum. In some embodiments, the glass sheets haveantireflection coatings on at least one side of the glass sheet.Suitable anti-reflective coatings include fluoropolymers.

Suitable polymer sheets can be single layers of a polyester film or afluoropolymer film, or can be multi-layer laminates comprising at leastone layer of a polyester film and at least one layer of a fluoropolymerfilm bonded together by an adhesive. In some embodiments, at least onepolymer sheet further comprises a layer of a metal, metal oxide ornon-metal oxide.

Typically, the top sheet is transparent to solar radiation.

Leads are attached to the top and bottom conducting layers. Typically,Mo is the bottom conductive layer and a Ni/Al grid is the top conductivelayer. These leads allow connection of the PV cell into a modulestructure.

EXAMPLES

The methods described herein are illustrated in the following examples.From the above discussion and these examples, one skilled in the art canascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various uses andconditions.

The following materials were used in the examples. All commercialreagents were used as received. Binderless mica paper (76.2 microns (3mils) thick was obtained from Corona Films (West Townsend, Mass., USA).Tetraethylorthosilicate (TEOS) was obtained from Sigma-Aldrich (St.Louis, Mo.). Tyzor® NBZ (zirconium tetra n-butanoate, 12 wt % inn-butanol), Tyzor® NPZ (zirconium tetra n-propanoate, 28 wt % inn-propanol), and Tyzor® TPT (neat tetraisopropyltitanate) were obtainedfrom Dorf Ketal Specialty Catalysts, LLC, a subsidiary of Dorf KetalChemicals PVT. LTD. (Mumbai, India).

The coefficient of thermal expansion of the mica paper composites wasmeasured by thermomechanical analysis (TMA). A TA Instrument (Model2940) was set up in tension mode with a N₂ purge of 30-50 mL/min rateand a mechanical cooler. The mica paper composite was cut to a 2.0 mmwidth in the machine direction and clamped lengthwise between the filmclamps, allowing for the measurement to be performed over a 7.5-9.0 mmlength. The preload tension was set for 5 grams force. The mica papercomposite was then heated from 0 to 400° C. at 10° C./min with a 3minute hold after reaching 400° C. It was cooled to 0° C. and reheatedto 400° C. at the same rate.

Tear initiation resistance (Grave's tear resistance) measurements wereperformed according to the procedures described in IPC-TM-650, number2.4.16 (The Institute for Interconnecting and Packaging ElectronicCircuits, Northbrook Ill.). An average of three measurements was usedfor each sample.

The following abbreviations are used: “C” is Celsius, “mm” ismillimeter, “mL” is milliliter, “min” is minute(s), “d” is day(s), “cm”is centimeter, “g” is gram(s), “mg” is milligrams, “wt” is weight, “wt%” means weight percent, “h” is hour(s), “temp” or “T” is temperature.

Example 1

A 10.2 cm×20.3 cm (4 inch×8 inch) piece of binderless mica paper (76.2microns thick, 3 mils) was taped onto a glass plate.Tetraethylorthosilicate (TEOS) was loaded into a 5 mL pipette and pouredacross a draw-down bar set for 381 micrometers (15 mil) clearance anddrawn across the paper. Typically, there was a slight excess of theliquid alkoxide which rolled off of the film. The alkoxide-treated micapaper was suspended and allowed to dry in air at room temperature forapproximately 60 minutes, then calcined in a furnace (Thermolyne, F6000box furnace) by heating to 600° C. at a rate of 4° C./minute in flowingair and holding at 600° C. for 60 minutes to provide a mica papercomposite. The weight of the alkoxide decomposition products, obtainedfrom gravimetric analysis of 1″×1″ coupons after heating to 600° C. for60 minutes, was 3.1 wt based on the total weight of the calcined micapaper composite. For these gravimetric measurements, a control sample ofmica paper (Comparative Example A) was also heated to 600° C. The weightdifference between the control and the 1″×1″ coupon of Example 1 wasused to determine the weight difference and the wt % in the final micapaper composite. Duplicate measurements were made to determine the finalweight percent values of the mica paper composite of Example 1.

No change in appearance was noted in the mica paper composite afterseveral hours of immersion in distilled water in a sealed vial,indicating greatly increased stability in water as compared with themica paper control of Comparative Example A and the calcined mica papercontrol of Comparative Example B.

The thermal expansion coefficient of the mica paper composite for thesecond heating cycle was measured as 6.2 ppm/° C. from 60° C. to 400° C.and 6.3 ppm/° C. from 60° C. to 350° C. This coefficient of thermalexpansion closely matches that of a Mo back electrode, which is about 5ppm/° C. in this temperature range, and is a characteristic desired of aphotovoltaic cell substrate.

The average force to initiate a tear was found to be about 836 g-f(grams-force), which is higher than the tear initiation resistance ofComparative Example A. This data is shown in Table 1.

Example 2

The same procedure was used as described in Example 1, except thatTyzor® NBZ (zirconium tetra n-butanoate, 12 wt % in n-butanol) was usedin place of TEOS. The alkoxide was loaded into a 5 mL pipette and pouredacross a draw-down bar set for 381 micrometers (15 mil) clearance anddrawn across the paper. Typically, there was a slight excess of theliquid alkoxide which rolled off of the film. The weight % of thealkoxide decomposition products, derived from gravimetry on 1″×1″coupons after heating to 600° C. for 60 minutes, was 3.6 wt % based onthe total weight of the mica paper composite.

No change in appearance was noted in the mica paper composite afterseveral hours of immersion in distilled water in a sealed vial,indicating greatly increased stability in water as compared with thecontrols of Comparative Examples A and B. The thermal expansioncoefficient for the second heating cycle of the mica paper composite wasmeasured as 4.7 ppm/° C. from 60° C. to 400° C. and 5.0 ppm/° C. from60° C. to 350° C. This coefficient of thermal expansion closely matchesthat of a Mo back electrode, which is about 5 ppm/° C. in thistemperature range, and is a characteristic desired of a photovoltaiccell substrate. The tear initiation resistance was measured to be 1100grams-force, as shown in Table 1. This value is much larger than thetear initiation resistance of Comparative Example A.

Example 3

The same procedure was used as described in Example 1, except thatTyzor® NPZ (zirconium tetra n-propanoate, 28 wt % in n-propanol) wasused in place of TEOS. The alkoxide was loaded into a 5 mL pipette andpoured across a draw-down bar set for 381 micrometers (15 mil) clearanceand drawn across the paper. Typically, there was a slight excess of theliquid alkoxide which rolled off of the film. The weight % of thealkoxide decomposition products, derived from gravimetry on 1″×1″coupons after heating to 600° C. for 60 minutes as described for Example1, was 8.1% based on the total weight of the mica paper composite.

No change in appearance was noted in the mica paper composite afterseveral hours of immersion in distilled water in a sealed vial,indicating greatly increased stability in water as compared with thecontrols of Comparative Examples A and B. The thermal expansioncoefficient of the mica paper composite for the second heating cycle wasmeasured as 6.0 from 60° C. to 400° C. and 6.1 ppm/° C. from 60° C. to350° C. This coefficient of thermal expansion closely matches that of aMo back electrode, which is about 5 ppm/° C. in this temperature range,and is a characteristic desired of a photovoltaic cell substrate.

The average force to initiate a tear was found to be about 747 g-f. Thisis higher than the tear initiation resistance of the mica paper controlof Comparative Example A.

Example 4

The same procedure was used as described in Example 1, except thatTyzor® TPT (tetraisopropyltitanate) was used in place of TEOS. Thealkoxide was loaded into a 5 mL pipette and poured across a draw-downbar set for 381 micrometers (15 mil) clearance and drawn across thepaper. Typically, there was a slight excess of the liquid alkoxide whichrolled off of the film.

No change in appearance was noted in the mica paper composite afterseveral hours of immersion in distilled water in a sealed vial,indicating greatly increased stability in water as compared with thecontrols of Comparative Examples A and B. The thermal expansioncoefficient of the mica paper composite for the second heating cycle wasmeasured as 6.6 ppm/° C. from 60° C. to 400° C. and 6.6 ppm/° C. from60° C. to 350° C. This coefficient of thermal expansion closely matchesthat of a Mo back electrode, which is about 5 ppm/° C. in thistemperature range, and is a characteristic desired of a photovoltaiccell substrate.

The average force to initiate a tear was found to be about 896 g-f. Thisis higher than the tear initiation resistance of the mica paper controlof Comparative Example A, as shown in Table 1.

Comparative Example A

Binderless mica paper (76.2 microns thick, 3 mils) as received fromCorona Films was used as a control comparison.

The thermal expansion coefficient for the second heating cycle of theuntreated mica paper was measured as 4.7 ppm/° C. from 60° C. to 350° C.Compared with the mica paper composites of Examples 1, 2, 3 and 4, theuntreated mica paper of the Comparative Example has essentially asimilar coefficient of thermal expansion. When immersed in distilledwater in a sealed vial, the untreated control sample of mica paperre-dispersed and was unstable in water after approximately 3-5 minutesat room temperature, indicating that it cannot be used in an aqueousprocess, such as a process to deposit cadmium sulfide or other n-typesemiconductors by aqueous routes.

Tear initiation resistance (Grave's tear) measurements were performedaccording to the procedures described in IPC-TM-650, number 2.4.16 (TheInstitute for Interconnecting and Packaging Electronic Circuits,Northbrook Ill.). An average of three measurements was used. The averageforce to initiate the tear was found to be about 625 g-f. This is lowerthan the tear initiation resistance of Examples 1, 3 and 4.

Comparative Example B

The untreated mica paper of Comparative Example A was heated in afurnace (Thermolyne, F6000 box furnace) by heating to 600° C. at a rateof 4° C./minute in flowing air and holding at 600° C. for 60 minutes toprovide a calcined mica paper.

When immersed in distilled water in a sealed vial, the calcined,untreated mica paper redispersed and was unstable in water afterapproximately 3-5 minutes at room temperature, indicating itsinstability in water. In contrast, the mica paper composites of Examples1, 2 3 and 4 showed stability in water for several hours at roomtemperature.

TABLE 1 Initial Tear Resistance (Tear Strength) Initial Tear Resistancestandard (grams-force) deviation Comparative Example A 623 172 Example 1836 151 Example 2 1100 292 Example 3 747 270 Example 4 896 136

1. A mica paper composite comprising a phyllosilicate component having ageneral Formula (V):[KMg₃(OH)₂[Si₃Al O₁₀]]_(j)[M^(m)O_((n-g))(OH)_(g)]_((1-j))  (V)wherein: 1) M is selected from Si, Al, Ti, Zr, and mixtures thereof,with the proviso that: i) when M is Si, Ti, or Zr, the value of m is 4;and ii) when M is Al, the value of m is 3; 2) n is a value representedby m/2; 3) g and (n-g) represent molar proportions, where g is a valuein the range of 0 to n; 4) j represents the weight fraction of mica inthe mica paper composite; and, 5) the value of j is in the range of0.995 to 0.8.
 2. The composite of claim 1 wherein the phyllosilicate isselected from muscovite, phlogopyte, illite, zinnwaldite, lepidolite,paragonite, biotite, fluorophlogopite, or combinations thereof.
 3. Thecomposite of claim 2 wherein the phyllosilicate is selected frommuscovite, illite, or combinations thereof.
 4. The composite of claim 1wherein M is selected from Si, Ti, Zr, or combinations thereof.
 5. Thecomposite of claim 4 wherein M is Ti, Si, or combinations thereof. 6.The composite of claim 1 wherein M is Zr.